THIS INVENTION relates to the treatment of water. More particularly, the invention relates to a process for the treatment of water which is acidic and contains dissolved ferrous (Fe2+) cations, optionally in association with dissolved sulphate (SO42xe2x88x92) anions.
The Applicant is aware of relevant prior art constituted by U.S. Pat. Nos. 5,427,691, 3,738,932, CH-A-590 791 and Patent Abstracts of Japan, Volume 009, No. 222 (1985) and Database WPI, AN 85-1501615 and JP-A-60 084196.
In U.S. Pat. No. 5,427,691 processes are disclosed for the treatment of acidic waters containing dissolved Fe2+ cations by means of aeration to oxygenate the water and by raising the pH of the water in the presence of suspended particulate material, to oxidize the Fe2+ cations to Fe3+ cations and precipitate them as Fe(OH)3. Lime is employed to raise the pH of the water to above 7 and the aeration takes place with the water at a pH above 7. A prior art process is also discussed in U.S. Pat. No. 5,427,691, in which limestone is used to remove Fe3+ cations. To remove Fe2+ cations using limestone, however, the Fe2+ cations must first be oxidized to Fe3+ cations, and doing so at acidic pH levels with air (dissolved oxygen) is described as xe2x80x98almost impossiblexe2x80x99 because of the slow reaction rates.
In U.S. Pat. No. 3,738,932, similarly, a process is disclosed for the treatment of acidic waters containing dissolved Fe2+ cations by means of aeration to oxygenate the water and by raising the pH in the presence of suspended particulate material, to oxidize the Fe2+ cations and precipitate them as Fe(OH)3. In this case, likewise, lime is used to raise the pH of the water to above 7 and the aeration takes place with the water at a pH above 7. No se of limestone or dolomite is described.
In CH-A-590 791 and Patent Abstracts of Japan supra, it is disclosed that it is known to use the oxidizing action of specific bacteria to oxidize Fe2+ cations to Fe3+ cations; and in said Patent Abstracts of Japan a two-stage process is disclosed whereby acidic sulphuric acid-containing waste water containing high concentrations of Fe2+ cations is subjected, in a first stage, to a bacterial oxidation wherein the Fe2+ cations are bacterially converted to Fe3+ cations. Then, in a second stage, calcium carbonate is added to the water which has been subjected to the bacterial oxidizing treatment, to precipitate Fe(OH)3.
According to the invention there is provided a process for the treatment of raw water containing dissolved Fe2+ cations and dissolved H+ cations so as to reduce the concentration of Fe2+ cations therein, the process comprising the oxidation of dissolved Fe2+ cations and the formation in the water of solid Fe(OH)3 from said Fe3+ cations, the process comprising the steps of:
oxygenating the water to achieve a dissolved oxygen concentration in the water of at least 0.1 mg/l, effective to oxidize the Fe2+ cations to Fe3+ cations; and
raising the pH of the water, the oxidation of the Fe2+ cations and the formation of the Fe(OH)3 being carried out in the presence of suspended particulate material in the water, the particulate material being present in the water at an effective concentration of at least 5 g/l,
the process being characterized in that, in combination,
the raising of the pH acts partially to neutralize the water;
the raising of the pH is by dissolving limestone or dolomite in the water; and
the dissolving of the limestone or dolomite in the water and the oxidation by dissolved oxygen of the Fe2+ cations to Fe3+ cations in the water take place together in the same body of partially neutralized water in the presence of the particulate material.
The process may comprise an agitation step whereby the water is agitated as it undergoes the oxygenating and the raising of its pH. Agitating the water may be by fluidizing it in a fluidized bed, fluidized eg by upward flow of air or oxygen through a body of the water containing the particulate matter, the particulate matter comprising particulate matter, such as calcium carbonate, added to neutralize the water, particulate matter precipitated from the water such as ferric hydroxide or gypsum, and/or slimes added to the water as a microorganism support. Instead, agitating the water may be by upward flow of the water through a fixed bed of particulate matter, or a packed bed of a support medium of the type described hereunder, conveniently at turbulent flow rates. Instead, the water may be mechanically agitated, so as to provide a fully-mixed or completely-mixed body of water, which again may be turbulent. Instead, the process can be carried out in a pipe or tube, along which turbulent flow takes place; and the water may be circulated through said beds or along said pipe or tube by means of a pump.
The water to be treated, i.e. the raw water, will typically have a Fe2+ cation concentration of at least 100 mg/l, usually 150-5000 mg/l and more usually 200-4000 mg/l; and will typically have a pH of at most 7, usually 1-6 and more usually 2-5. Often, the water to be treated will also contain SO42xe2x88x92 ions at a concentration of at least 200 mg/l, usually 200-25 000 mg/l, and more usually 1000-10 000 mg/l. This water will typically have an Acidity, i.e. an HCOxe2x88x923 acidity, expressed as mg/l of CaCO3, of 200-30 000, usually 400-25 000 and more usually 1000-10 000.
Oxygenating the water may be such as to achieve a dissolved oxygen concentration in the water of at least 0.5-8 mg/l, the suspended solid material having a particle size distribution whereby at least 50% by mass thereof has a particle size of less than 500 xcexcm, the particulate material being present in the water at a concentration of at least 10 g/l, the raw water having a dissolved Fe2+ cation concentration of more than 100 mg/l, and the process acting to to decrease the dissolved Fe2+ cation concentration to less than 100 mg/l.
More particularly, the raw water may contain dissolved SO42xe2x88x92 anions, the process including the biological oxidation of dissolved Fe2+ cations to Fe3+ cations and the process being carried out at a temperature of 0-90xc2x0 C., preferably 5-40xc2x0 C. In this case the raw water may have a dissolved SO42xe2x88x92 anion concentration of at least 200 mg/l, the biological oxidation being carried out by microorganisms selected from
Ferrobacillus ferrooxidans; 
Ferrobacillus thiooxidans; 
Thiobacillus thiooxidans; and 
mixtures of any two or more thereof.
The microorganisms may be supported on a support medium, to increase the concentration of microorganisms in the agitated water. While the support medium may be of metal or synthetic plastics material, such as rings, plates (which may be corrugated) and superimposed corrugated plates such as those available in South Africa under the Trade Mark TERBO PLASTIC, to provide a surface area for microorganism growth of at least 10 m2 of support medium area/m3 of agitated water, preferably 100-1000 m2/m3 and more preferably 200-500 m2/m3, the support medium instead is conveniently a packed or suspended particulate material, in particular a finely divided particulate material, such as a slimes or sludge material or sediment added to the agitated water either continuously, or at the start of the process to be progressively supplanted by solids produced by the process as the process proceeds. The particulate material present in the agitated water may have a particle size of at most 500 xcexcm, preferably 5-200 xcexcm, and more preferably 10-100 xcexcm. This particulate material may be present at a concentration of 10-500 g/l, preferably 50-200 g/l; and the particulate material may provide a particle surface area in the agitated water of at least 100 m2/m3 of agitated water, preferably 100-10 000 000 m2/m3. Examples of suitable particulate materials for initial employment are waste coal fines and gypsum (when sulphate is precipitated by calcium salts addition as described hereunder), and will be progressively supplanted by precipitated Fe(OH)3 optionally admixed with gypsum (CaSO4.2H2O) if calcium salts (as described hereunder) are used for the neutralization, and if the water to be treated contains SO42xe2x88x92 anions. If desired, both a support medium in the form of rings or plates, as described above, can be used, and a particulate support medium, such media both acting to provide an increased surface area for growth of microorganisms.
The raw water may have a pH of at most 7, the raw water having an Acidity, expressed as mg/l of CaCO3 dissolved therein, of at least 200, and the suspended particulate material providing a particle surface area in the water of at least 100 m2/m3 water. When the raw water contains dissolved SO42xe2x88x92 anions, raising the pH of the water may comprise adding CaCO3 thereto to cause the formation of solid CaSO4.2H2O in the water, the CaSO4.2H2O being allowed to precipitate from the water. In this case, the oxidation of dissolved Fe2+ cations to Fe3+ cations, the addition of CaCO3 to the water and the precipitation of CaSO4.2H2O from the water will take place together in the same body of water.
The process may be carried out at ambient temperatures such as the temperature of 0-90xc2x0 C. as mentioned above, preferably 5-40xc2x0 C. and more preferably 15-30xc2x0 C. While the process can in principle be carried out on a batch basis, it is conveniently carried out on a continuous basis; and it may be carried out in a single stage or in a plurality of stages arranged in parallel and/or series. When the process is in operation, any biological oxidation of Fe2+ to Fe3+ tends to cause a drop in pH, so that neutralization must be effected continuously or intermittently, so as to keep the pH up to a desired level of at least 3, eg above 4, preferably above 5 and more preferably above 6. Typically the water will be treated for an average period or reaction time of at least 1 minute, usually 20-1440 minutes and more preferably 30-480 minutes, which will be its average residence time in a single stage when a single stage is used or its total residence time in a plurality of stages, when a plurality of stages is used.
At least partially neutralizing the water may be by adding a suitable base or alkali, optionally in particulate form, thereto, examples of suitable alkalis being CaCO3, Ca(OH)2, CaO and NaOH, in particular limestone or calcium carbonate (CaCO3), but not excluding dolomite or waste alkalis obtainable in mixed form as steel industry waste products. The alkali added preferably has a particle size of at most 500 xcexcm, more preferably at most 100 xcexcm and conveniently as small as practicable, bearing economic considerations in mind.
Oxygenating the water may be by feeding oxygen to the water, and this may be by bubbling oxygen or conveniently air through the water. The feed rate may be such as to achieve a preferred dissolved oxygen content in the water of 1-5 mg/l.
In accordance with the invention water may be treated continuously by the process and may pass on to a settling or sedimentation stage where metal hydroxides or oxides, in particular Fe(OH)3, optionally containing CaSO4.2H2O, will be precipitated therefrom, and slimes or sediment from this sedimentation stage may be recirculated to the process to give the particulate material which provides the surface area which promotes the oxidation of Fe2+ to Fe3+. This particulate material can also support the microorganisms which carry out the biological oxidation of the Fe2+ to Fe3+.
In terms of a variation of the process, treated water flowing from the oxidation of Fe2+ to Fe3+ to the sedimentation stage may be used to dispose of acidic waste water with a pH of 2-6. Such acidic waste water may have a pH of 3-4, and an Fe2+ cation concentration of at least 100 mg/l, usually 100-2000 mg/l, and more usually 100-800 mg/l; it may have an Acidity, expressed as mg/l of CaCO3 of at least 200, usually 200-4000 and more usually 200-1600; and it may have a SO42xe2x88x92 anion concentration of at least 200 mg/l, usually 200-4000 mg/l and more usually 200-2600 mg/l. This can be effected by mixing the water issuing from the Fe2+ oxidation stage with the acidic waste water and dosing it with a strong alkali such as lime (CaOH) to raise its pH to a value of 6-9, eg about 7, eg in an aeration tank. This may be done by first adding the strong alkali in a conditioning tank to water or sludge recirculated from the sedimentation stage, to raise its pH to 11-12, this water, at a pH of 11-12, being used in the aeration stage to raise the pH, after which, in the sedimentation stage, a suitable flocculant may be used to promote settling of the Fe(OH3) and optionally of the CaSO4.2H2O.
Accordingly, after the formation of the Fe(OH)3, the water may be subjected to a sedimentation step to settle suspended solids therefrom. In particular, the water may be treated on a continuous basis, the sedimentation step being carried out separately from the oxygenating of the water and separately from the raising of the pH thereof, solids settled by the sedimentation step being recirculated and the oxidation of the Fe2+ cations taking place in the presence of the recirculated solids. In this case, the oxygenation of the water may include an aeration step, separate from the step of raising the pH of the water, solids settled by the sedimentation step being recirculated to the pH-raising step and to the aeration step.
The oxidation reactions which take place according to the process can be expressed by:
2Fe2++xc2xdO2+2H+xe2x86x922Fe3++H2O;
and
2Fe3++6H2Oxe2x86x922Fe(OH)3↓+6H+,
and, when SO42xe2x88x92 is present in the water to be treated by the biological oxidation and a calcium salt is used for neutralization of that water, gypsum is produced according to the chemical reaction:
2H2O+Ca2++SO42xe2x88x92xe2x86x92CaSO4.2H2O↓.
When weakly acid waste water of a pH of above 4 is treated according to the process of the invention, the treated water may have, in combination, a pH of 5.0 or more, an Fe2+ cation content of 100 mg/ l or less, an acidity as mg/l of CaCO3 of 500 or less, and a SO42xe2x88x92 anion content of 4000 mg/ or less. When the process is used also to dispose of highly acid waste water with a pH of less than 4, the treated water may have a pH of 4.0 or more, an Fe2+ cation content of 100 mg/l or less, an Acidity as mg/l of CaCO3 of 1000 or less, and a SO42xe2x88x92 anion content of 4000 mg/l or less, typically less than 3000 mg/l.